Diffraction type sound absorber covered by a membrane



March 28, 1950 H. F. OLSON 2,502,018

DIFFRACTION TYPE SOUND ABSORBER COVERED BY A MEMBRANE Filed March 30, 1944 IN VEN TOR.

Ha If? 1 011012 BY [A z ATTORNEY desired acoustical characteristics.

Patented Mar. 28, 1950 UNITED STATES PATET OFFICE DIFFRACTION TYPE SOUND ABSORBER, COVERED BY A MEMBRANE Harry F. Olson, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware 9 Claims.

This invention relates to acoustic absorbers and more particularly to acoustic absorbers of the diffraction type, the present invention having particular reference to the type of acoustic ab sorber described and claimed in my copending application Serial No. 512,320, filed November 30, 1943.

In the aforesaid copending application, there is disclosed a diffraction type acoustic absorber comprised of a casing which encloses a relatively large volum of air and the wall structure of which is constituted by a material which is pervicue to sound waves but which offers a high dissipative impedance thereto. By making the wall structure of such a material and making the cavity or space within the casing so large that the volume of air therein offers a very small im pedance to the sound waves, a very highly effi, cient diffraction type absorber is provided.

According to one form of acoustic absorber disclosed in my aforesaid copending application, the wall structure of the unit is made out of a felted material formed by accretion of fibres from a liquid suspension thereof. This material may be formed in the manner more particularly disclosed and claimed in the copending application of Victor T. Par, Serial No. 519,879, filed January 27, 1944, now Patent No. 2,488,555, granted on November 22, 1949 and assigned to the Radio Corporation of America, but any other type of felted product may be used provided it has the For example, in one type of acoustical absorber according to my prior invention, the wall structure of its casing is constituted by about 93.5 percent of shredded wood fibers and about 6.5 percent bagasse as a binder, the wall having a thickness of about inch.

While sound absorbers of this type are highly efiicient in use, the felted type of material is open to the objection that it sheds lint or fibers easily, since the fibers are more or less easily separable from the material. This objection is not serious in the case of absorbers which are to be installed in factories and the like, but for use in hospitals. laboratories, homes, etc, where dust, lint and the like are highly objectionable, such absorbers are not entirely satisfactory.

The primary object of my present invention, therefore, is to provide an improved form of diffraction type acoustic absorber which will be free from the aforementioned objection.

More particularly, it is an object of my present invention to provide an improved diffraction type acoustic absorber which, while made of fibrous 2 material, will, nevertheless, be free from shedding the fibers thereof.

Another object of my present invention is to provide improved diffraction type acoustic absorber which will not contaminate the surroundings.

It is also an object of my present invention to provide an improved diffraction type acoustic absorber which is simple in construction, economical in cost, and highly efficient in use.

In accordance with my present invention, I place around the Wall structure of the sound absorbing unit or body a covering of some suitable sheet material which is impervious to lint, dust, and other similar dust-like matter, but which is transparent to sound waves. The material must be very light and flexible so that its acoustic impedance will b small compared to the characteristic acoustic impedance of air. For this purpose, thin sheeting of metal, such as aluminum, or of various plastic materials may be used, these materials having a thickness not in excess of 1 1000 inch. A thin sheet material of this type is not suitable for covering conventional sound absorbing materials used as wall structuresbecause, in large sheets, it is very fragile. Moreover, difficulty is encountered at the joints of such sheets. However, it is perfectly feasible to cover the outside of a diffraction type acoustic absorber or acoustic sink with such thin, sheet materials because the acoustic sink is small and a complete unit in itself. Furthermore, crinkling or dents in the covering over a diffraction type absorber would not be noticeable. Preferably, the outer surface of the wall structure of the acoustic sink is formed with ridges or other suitable projections which may be felted on the surface thereof for spacing the covering from the remainder of the wall structure so that the stiffness of the covering will be reduced. This precaution may be taken to insure proper absorption of low frequency sounds by the unit.

The novel features that I consider characteristic of my invention are set forth with particularit in the appended claims. The invention itself, however, both as to its organisation and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description of two embodiments thereof, when read in connection with the accompanyin drawing in which Figure l is a top plan view of one form of acoustic sink or absorber constructed in accord-- ance with my present invention,

Figure 2 is a sectional view thereof taken on the line 11-11 of Fig. 1, and

Figure 3 is a perspective view, partly broken away, of a somewhat different form of acoustic absorber according to my present invention.

Referring more particularly to the drawing, there is shown, in Figs. 1 and 2, an acoustic absorber l constituted by two hollow casing halves Ia and lb, each formed as a cone. These cones may either be nested one within the other for storage, or they may be assembled, as shown in Fig. 2, to enclose a relatively large air filled space 3. The cones are preferably formed by felting fibers of suitable material into a more or less compact relation, the fibers extending in random directions and providing a plurality of intertwined, thin slits or passages through which the space 3 has communication with the exterior of the casing. These passages are pervious to sound waves but oiier a large dissipative impedance thereto, whereas the space .5 is of such volume that the capacity thereof is large enough to pro-- vide a very small. impedance to the incoming sound waves. The acoustic absorbing unit i thus far described constitutes the subject matter of my aforementioned copending application and may be suspended from the ceiling or mounted on suitable supports in spaced relation to walls and ceiling to receive sound Waves from all directions.

Since the wall structure of the absorber i is made of a felted material, it will be clear that the fibers thereof will be or less readily separable therefrom and, when loose, will fall to the floor or will be carried on by air currents and thus contaminate the surroundings. To prevent this, the cones la and lb are formed with a plurality of spaced ridges, corrugations, or other suitable projections E3 on the outer surfaces thereof, and over these projections is provided a covering '1 of thin, sheet material having a thickness preferably less than, but not over about /1000 inch. The covering i may be made of metal, rubber, plastic materials such as vinyl resin, cellophane, etc., it being essential only that the material shall be transparent or pervious to sound without substantially attenuating the sound waves and that it be impervious to dust-like particles, such as the fibers of which the wall structure is made, etc. The covering i is in contact with the wall structure at the ridges or the like 5 and is therefore spaced from the greater part of the casing, whereby the stiffness of the covering is maintained at a minimum. Sound waves incident on the covering "l cause it to vibrate and to transmit vibrations through the wall struc ture of the absorber l where the sound energy is dissipated.

In Fig. 3 is shown a similar acoustic absorber or sink ll of parallelepiped form. The wall structure of the sink ii is made of a fibrous material and encloses a large air space IS. The longitudinally extendin ridges i5 thereon maintain the sound pervious but dust-or-the-lil e impervious covering ii in spaced relation to the greatest part of the wall structure.

The improved acoustic absorbers described above and having suitable coverings over the wall structures thereof are not only highly effective as absorbers, but will not contaminate the surroundings due to fibers which become loose, since such loose fibers are confined within the covering. Although I have shown and described two forms of my present invention, it will undoubted- 1y be apparent to those skilled in the art that many other variations thereof, as Well as changes in the ones described herein, are possible. I therefore desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A diffraction type acoustic absorber compris ing a self -contained unit adapted to be suspended in space and constituted by a casing itself enclosing a fluid filled space, said casing having a wall structure which is pervious to acoustical waves but which ofiers a relatively large dissipative impedance to the passage of acoustical wave energy therethrough, and a covering over said wall structure, said covering being impervious to dust-like particles but transparent to acoustical waves.

2. A. diifraction type acoustic absorber comprising a self -contained unit adapted to be suspended in space and constituted by a casing itself enclosing a fiuid filled space, said casing having a wall structure which is pervious to acoustical waves but which offers a large dissipative impedance to the passage of acoustical wave energy therethrough, and a membranous covering over said wall structure, said covering being impervious to dust-like particles but transparent to acoustical waves.

3. A diifr ction type acoustic absorber comprising a self-contained, closed casing adapted to be suspended in space and enclosing an air filled space therein, said casing having a wall structure which is pervious to acoustical waves but which offers a large dissipative impedance to the passage of acoustical wave energy therethrough between the exterior of said casing and said space, said space having a volume such that the air therein offers a relatively small impedance to the passage of said waves through said wall structure, and a membranous covering over said wall structure, said covering being impervious to dust-like particles but transparent to acoustical waves.

4. A. diffraction type sound absorber comprising a self-oontained unit adapted to be suspended in space and having an internal. cavity bounded by a wall structure, said cavity containing a compressible fluid and said wall structure having a plurality of passageways therein which establish communication between said cavity and the exterior of said body for the passage of sound waves therethrough, said passageways offering considerable dissipative impedance to the passage of said waves and said cavity having a volume such that the fluid therein offers a relatively small impedance to the passage of said waves through said passageways, and a membranous covering over said wall structure, said covering being impervious to dust-like particles but being pervious to sound waves.

5. A diffraction type sound absorber body according to claim 4 characterized. in that said membranous covering has an acoustical imped ance which is small compared to the characteristic acoustic impedance of air.

6. A diffraction type sound absorber according to claim i characterized in that said membranous covering is of sheet material having a thickness not in excess of /1000 inch.

7. A diffraction type sound absorber according to claim 4 characterized in that said wall structure is constituted by superposed layers oi fibers extending in random directions, said fibers being more or less easily separable from said wall structure, and said covering serving to confine to said body any fibers which may come loose from said wall structure.

8. A diffraction type sound absorber according to claim 4 characterized in that said Wall structure is provided with a plurality of projections on the outer surface thereof, and characterized further in that said covering contacts said Wall structure at said projections.

9. A diffraction type sound absorber according to claim 4 characterized in that said Wall structure is provided with a plurality of ridges on the outer surface thereof and characterized further in that said covering contacts said Wall structure at said ridges but is elsewhere spaced from said Wall structure whereby to maintain the stiffness of said covering at a minimum.

HARRY F. OLSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

